Are there guidelines over what is considered “Eligible” sound recordings that can be monetized and “Ineligible” sound recordings that can’t be monetized?

YouTube automatically generates claims against other user's uploaded content that matches the music you provide, so it very important that the music you are opting in is eligible to avoid falsely claiming other user’s content. This rule applies to Facebook as well but their program will show no visual indicator (like a claim or ad).

Eligibility:

Songs that you control all of the master and publishing rights to.

Use only your own material for which you have exclusive rights.

Use third-party material for which you have exclusive rights.

The following examples are Ineligible and cannot be used for YouTube or Facebook Monetization:

Content licensed non-exclusively from a third party (such as samples that are not exclusively licensed)